In a conventional fuel cell system, for example, of a phosphoric acid electrolyte-type, an alkali electrolyte-type, or a solid polymer electrolyte-type, natural gas, petroleum, or coal, which serves as a raw fuel, must first be gasified and reformed in an external gasifying-and-reforming reaction step by combining reforming reactions (I) and (II), which are endothermic reactions, with an exothermic reaction (III), and by converting all of the mixed gases of carbon monoxide and water by the shift reaction (IV). EQU C.sub.m H.sub.n +mH.sub.2 O.fwdarw.mCO+(m+n/2)H.sub.2 (I) EQU C.sub.m H.sub.n +mCO.sub.2.fwdarw.2mCO+n/2H.sub.2 (II) EQU C.sub.m H.sub.n +m/2O.sub.2.fwdarw.mCO+n/2H.sub.2 (III) EQU CO+H.sub.2 O.fwdarw.H.sub.2 +CO.sub.2 (IV)
In molten carbonate fuel cell systems and oxide fuel cell systems, which are high-temperature fuel cell systems, the shift reaction (IV) can be omitted. Nevertheless, in these fuel cell systems raw fuel is first converted to a mixed gas of hydrogen and carbon monoxide, by the reactions (I) to (III), which is then introduced into the fuel cell. Since in these conventional fuel cell systems, the step of gasifying and reforming the raw fuel and the fuel cell step are separate steps, the whole system is complicated. Therefore, there is a disadvantage in that the efficiency, for example, of heat recovery is low.
Further, when coal is used as a fuel, the operating conditions of the conventional coal gasifying-and-reforming step require a pressure and a temperature (above 1473 K) that are far higher than even those of solid oxide fuel cells, which operate at higher temperature than other types of fuel cells. Therefore, it is necessary that the coal gasifying-and-reforming step and the fuel cell step be separated into separate plants when making up a power-generation system. As a result, for example, it is not possible for the heat generated by solid oxide fuel cells to be recycled for effective utilization in the coal gasification process. Therefore, when coal is used as fuel, only a lower generation efficiency can be obtained in comparison with the use of natural gas.
Recently, a direct internal reforming method has been studied, wherein a separate gasifying-and-reforming reaction step is not required by incorporating the fuel gasifying-and-reforming step, which is an endothermic reaction, in the fuel cell stack in which an exothermic reaction takes place. This enables recovery of at least the heat loss that occurs with the reaction (III). In this direct internal reforming method, the activity of a conventional electrode, made up mainly of nickel, decreases significantly. In order to restrict this decrease in activity, it is necessary to add a large amount of water. However, because heat flows in and out with the evaporation and condensation of this added water, the efficiency of the whole system, including the efficiency of heat exchange, is considerably reduced.
Although natural gas has been studied with respect to the direct internal reforming method, hydrocarbon fuel, naphtha, and coal, which have higher molecular weights, have not yet been studied. This is because with the use of long-chain hydrocarbons, such as naphtha, in the direct internal reforming method, deposition of carbon is significant. As a result, a conventional nickel fuel electrode is "poisoned" and deactivated even if a large amount of steam is introduced. Therefore, since the carbon ratio of coal is even higher than that of naphtha, the direct internal reforming method using coal as a raw fuel has been considered impossible to pursue.
As discussed above, a coal gasifying apparatus and a fuel cell are combined into a complex system, which requires the use of a fuel cell into which coal can be directly introduced or a fuel cell that can be installed in a coal gasifying apparatus. Further, as a fuel electrode for such a fuel cell, a fuel electrode made of a new material is required. This material must not be "poisoned" or deactivated, like the above-mentioned nickel electrode, even when it comes in direct contact with coal. However, there are no conventional fuel electrodes that satisfy this requirement.